This application is a competitive renewal of a project directed at furthering the understanding of the basic scattering mechanisms of ultrasound in biological tissues, particularly in blood. In the next grant period, we propose to carry out the following 3 tasks. (1) We will continue the investigation of ultrasonic scattering properties of whole blood and erythrocyte suspension under well-defined flow conditions with more emphasis on pulsatile flow both in vitro in a mock flow loop and in vivo on pits. (2) We will extend both the in vitro scattering measurements on blood and other soft tissues including muscle, liver, and kidney to 60 MHZ using a newly developed method. (3) We will also investigate a new method for quantitating the amount of specular reflection in tissues which may lead to the further development of ultrasonic tissue characterization, a goal that has not yet been achieved. The rationale for pursuing Tasks #1 and 2 is that this knowledge is needed for the better interpretation of ultrasonic B-mode and Doppler images and the further development of this imaging modality. Recently there has been a growing devices which typically are operated at frequencies higher than 20MHz and Doppler power imaging. In the former application, it has been shown that frequencies approaching 40 MHZ have to be used in order to be able to clearly visualize structures in the blood vessel wall. As a result an understanding of high frequency blood scattering properties is of critical importance since echoes from blood may interfere with those from structures with the vessel wall at these frequencies. In the latter application, Doppler shift from blood is used to produce a color map to indicate tissue perfusion or blood supply. Clinical applications of this technique depend upon a clear understanding of the relationship between Doppler power from blood and hematological and hemodynamic parameters. Another rationale for pursing Tasks #2 is that although the next frontier of ultrasonic appears to be in a very high frequency (VHF) imaging in the range of 30 to 80 MHZ, data on attenuation and scattering properties of soft tissues in this frequency range are lacking apparently due to experimental difficulties including the unavailability of sensitive high frequency transducers and suitable methodology. We have here at Penn State University the capability of designing and fabricating high frequency transducers and have recently developed a method which enables scattering measurements to be made with focused transducers. The rationale for pursuing Task#3 is that the proposed dual frequency subtraction approach which allows the estimation of the amount of specular reflection in the returned echoes from tissues may be used to minimize the data variation in tissue characterization which has been a major problem in preventing ultrasonic tissue characterization from becoming a viable clinical tool.